1. Field of the Invention
This invention relates to a method for concentrating or liquefying a specified dilute gaseous component in a gaseous mixture.
2. Description of the Prior Art
According to conventional techniques, a specified dilute component gas in a gaseous mixture is concentrated either by (1) a vacuum or pressure-reduction method, or (2) a heating method. Method (1) relies on the use of an adsorbing device composed of a pair of adsorbing layers in which the absorption of a given gas is performed in one adsorbing layer while desorption of the adsorbed gas is performed in the other, with this sequence of operations being repeated.
The given gas adsorbed will naturally be desorbed from the adsorbing layer if the pressure is lower than the partial pressure of the given gas component in the starting gaseous mixture. However, because of the existance of adsorption energy, etc., the specified component cannot be desorbed to an extent such that the concentration of this component in the gas to be desorbed reaches an adsorption equilibrium concentration corresponding to the above-mentioned low pressure. For this reason, desorption is carried out while passing a carrier gas into the adsorbing layer so tht the conentration of the specified component becomes not more than the adsorption equilibrium concentration corresponding to the low pressure. The amount R of the carrier gas fed into the adsorption layer at this time can be determined by the following equation: ##EQU1## wherein .eta. is the efficiency coefficient, 1.2-10. P.sub.1 is the pressure of feeding the starting gaseous mixture,
P.sub.O is the reduced pressure, and PA1 D is the amount of the gas released. PA1 (1) passing a starting gaseous mixture containing a dilute gaseous substance to be selectively separated into one adsorbing layer of an adsorbing device consisting of a pair of adsorbing layers to permit the same to adsorb the gaseous substance to be selectively separated; PA1 (2) releasing the remainder of the gaseous mixture, and, before the amount of the adsorbed gas in the adsorbing layer reaches saturation, switching the passing of the starting gaseous mixture over to the other adsorbing layer; PA1 (3) maintaining the pressure of the inside of the adsorbing layer which has adsorbed the gaseous substance lower than the pressure of the starting gaseous mixture fed; PA1 (4) feeding a part of the gas released from the other adsorbing layer or an inert gas from outside the system to desorb the adsorbed gas; PA1 (5) again passing the starting gaseous mixture after the end of desorption and switching the desorption of the adsorbed gaseous mixture over to the other adsorbing layer; and PA1 (6) mixing the desorbed gas with the starting gaseous mixture either directly or after separation of the resulting liquefied gas, and recycling and passing the gaseous mixture through the adsorbing layer, wherein the recycling is repeated until the concentration of the specified component in the recycle gas to be passed into the adsorbing layer reaches the desired concentration or the component is liquefied, and the specified component having attained the desired concentration or the liquefied specified component is continuously recovered to maintain the gas recycling system in balance and steady state.
Since the degree of vacuum that can be used commercially in such a process is about 10 to about 160 Torror more, the starting gaseous mixture can be concentrated only to about 20 to 40 times. For example, when a starting gaseous mixture of 100 ppm is fed, there cannot be obtained a concentrated gas of more than 2000 to 4000 ppm, at P=760 Torr, P.sub.O =10 Torr and .eta.=2.
According to method (2) above, heated nitrogen gas or stream is used, but since this method does not involve a pressure reduction for desorption, it is impossible to obtain a gas which is concentrated to a greater extent than in the case of method (1).
U.S. Pat. No. 2,944,627 Skarstrom process involves conservation of heat evolved at the adsorption cycle, and very rapid cycling is required in the process.
The Skarstrom process does not involve the steps that the recycling is repeated until the concentration of the specified component in the recycle gas to be passed into the adsorbing layer reaches the desired concentration or the component is liquefied, and the specified component having attained the desired concentration or the liquefied specified component is continuously recovered to maintain the gas recycling system in balance and steady state.
U.S. Pat. No. 3,085,379 Kiyonaga et al related to removing impurities, whereas the present invention relates to recovering a particular component.
Referring to Column 4, line 25 et seq of Kiyonaga et al, it appears that compression is necessary, whereas such is not always necessary in the present invention. In Kiyonaga et al, the selection of the desorption stroke pressure is "critical as it determines purity of the gaseous product produced."; in this regard, a specific requirement is posed at Column 6, line 5 et seq. The present invention does not involve such a critical selection of a desorption stroke pressure.
U.S. Pat. No. 3,149,934 Martin required compressing the desorbate and recycling at least a portion thereof to the bottom of the adsorption bed. Further, the feed must be introduced at an intermediate point into the adsorption zone, i.e., at an equilibrium point. This is quite dissimilar in concept from the present invention. Reference to Martin shows that little attention is given to the pressure of desorption. Further, in Martin the zones Y1 and Y2 are saturated with the A component from a previous adsorbant step. Finally, if one refers to Column 4, lines 14 et seq of Martin, the concept of controlling the partial pressure of A is very important in Martin. Such partial pressure considerations are not of critical importance in the present invention.